Vref is defined as landing speed or the threshold crossing speed, while Vapp is defined as the approach speed with wind/gust additives.
LEFT: Final Approach: Finals using 'crab approach' (Airbus A320)
When landing with a headwind, crosswind, or tailwind the Vref and Vapp must be adjusted accordingly to obtain the optimal speed at the time of touchdown. Failure to do this may result in the aircraft landing at an non-optimal speed causing runway overshoot, stall, or floating (ground affect).
Mathematical calculations can be used to determine Vref and Vapp based on wind speed, direction, and gusts.
When using the autothrottle, position command speed to Vref+5 knots.
If the autothrottle is disengaged or is planned to be disengaged prior to landing, the recommended method of approach speed correction to obtain Vapp, is to add one half of the reported steady headwind component plus the full gust increment above the steady wind to the reference speed.
One half of the reported steady headwind component can be estimated by using 50% for a direct headwind, 35% for a 45-degree crosswind, zero for a direct crosswind, or interpolation between.
When making adjustments for wind additives, the maximum command speed should not exceed Vref+20 knots or landing flap placard speed minus 5 knots, whichever is lower.
The minimum command speed setting with autothrottle disconnected is Vref+5 knots. The gust correction should be maintained to touchdown while the steady headwind correction should be bled off as the airplane approaches touchdown.
It is important to note that Vref+5 knots is the speed that is desired when crossing the threshold of the runway - it is NOT the approach speed. The approach speed (Vapp) is determined by headwind with/without gusts. If the wind is calm, Vref+5 knots will equal Vapp.
When landing in a tailwind, do not apply wind corrections. Set command speed at Vref+5 knots (autothrottle engaged or not engaged).
When Vref has been adjusted by the non-normal procedure, the new Vref is called the adjusted Vref and is used for the landing. To this speed is added the wind component (if necessary).
For example, if a non-normal checklist specifies 'Use flaps 15 and Vref 15+10 for landing', the flight crew would select flaps 15 and look up the Vref 15 speed (in FCTM or QRH) and add 10 knots to that speed. The adjusted Vref does not include wind corrections and these will need to be added.
If the autothrottle is disengaged, or is planned to be disengaged prior to landing, appropriate wind corrections must be added to the adjusted Vref to arrive at command speed. Command speed is the safest speed used to fly the approach (Vapp). If the speed is above command speed then it will need to be bleed off prior to touchdown.
An interesting publication (powerpoint presentation) concerning the use of the autothrottle can be read here autothrottle usage - training alert. Search for Autothrottle Usage - Training Alert.
If using autoland (CAT II & CAT IIIA) the autothrottle remains engaged and the command speed is set to Vref+5.
The following autoland limitations must be abided by:
(i) Glideslope angle tolerance - maximum 3.25 degrees / minimum 2.5 degrees;
(ii) Engines 1/2 operational;
(iii) Maximum tailwind - 15 kts;
(iv) Maximum headwind - 25 kts;
(v) Maximum crosswind - 20 kts ;
(vi) Maximum taliwnd at flaps 30 - 12 knots (winglets); and,
(vii) Landing in gusty wind or windshear conditions is not approved during CAT II and CAT IIIA operations.
Guideline (an easy way to remember the above - cheat sheet)
This information assumes the autothrottle will be disengaged prior to landing.
- Headwind less than 10 knots: Vref+5
- Headwind greater than 10 knots: Vref + headwind / 2 (half your headwind) - This is your Vref
- If Vref is > 20 knots, then: Vref+20 (as per placard guide)
- Formula (Wind < 10 knots): Vref+5 + gust – headwind
- Formula (Wind > 10 knots): Vref + headwind/2 (half your headwind) + gust – headwind
Calculating Directional Wind
A wind component will not always be at 90 Degrees or straight on to your landing direction. The following calculation is often used to determine the directional component. One half of the reported steady headwind component can be estimated by using 50% for a direct headwind, 35% for 45 degree crosswind, zero for a direct crosswind and interpolation in between.
Tail winds are very challenging for conducting a stabilized approach. Because of the increased ground speed caused by a tail wind, Boeing does not publish Vref correction factors for tail winds.
Typically, to maintain the proper approach speed and rate of descent while maintaining glideslope, thrust must be decreased which minimizes the available safety envelope should a go-around be required. If a go-around is required, precious seconds might be lost as the engines accelerate; the aircraft would continue to descend and might touch down on the runway before the engines produce enough thrust to enable a climb.
The International Civil Aviation Organization (ICAO) recommends that the tail wind component must not exceed 5 knots plus gusts on a designated runway; however, adherence to this recommendation varies among members. Several airlines have been certified for operation with a 15 knot tailwind.
In the United States, Federal Aviation Administration (FAA) sets the tail wind component limit for runways that are clear and dry at 5 knots, and in some circumstances 7 knots, however FAA allows no tail wind component when runways are not clear and dry. Note, that many manuals subscribe to the 10 knots no tailwind rule (see table below).
Crosswind components can be variable dependent upon flight crew discretion and airline policy; therefore, the above is to be used as a 'guide' only.
The below table (limitations) summarizes much of what has been written above.
Using the CDU - wind component and wind correction field (WCF)
The CDU if configured correctly can provide information concerning wind components. Press the key on the CDU named 'PROG' followed by 'PREV PAGE'. This page provides an overview of the wind component including head, tail and crosswind.
The CDU in the approach page has a field called the Wind Correction Field (WCF). The flight crew can alter the Vref+ speed used by the autothrottle as desired. The default reading is +5.
To briefly explain the WCF. Boeing when they designed the auto throttle algorithm programmed a speed additive that the A/T automatically adds to Vref when the A/T is engaged. The reason for adding this speed is to provide a safety buffer to ensure that the A/T does not command a speed equal to or lower than Vref. Recall that wind gusts can cause the autothrottle to command %N1 to spool up or down depending upon the gust strength.
This said, during autoland operations the autothrottle is always set to Vref+5, and if disengaging the autopilot and autothrottle prior to landing, then the standard Vref calculation based on wind speed and direction is made. The WCF is a handy feature if a flight crew wishes to increase the safety margin the autothrottle algorithm operates.
A Vref +speed higher than +5 can be added when gusty conditions that are above what are considered normal. By increasing the +speed, the speed commanded by the autothrottle will not degrade to a speed lower than that inputted.
Remember, that during the approach V speeds are important to maintain and a commanded speed that is below optimal can be dangerous if the crew needs to conduct a go-around or if winds suddenly increase or decrease which can cause issues with pitch coupling.
This is part two of a two part post. Part one dealt with methods used to land in cross wind conditions - Crosswind Landing Techniques Part One Crab and Sideslip.